Amplifiers are widely used in electronic circuits. The amplifiers are popular because they have characteristics that are desirable for voltage amplification. The amplifier circuits are characterized by high input impedance and low output impedance that allow extremely small input currents, and hence low power, to drive the circuit and relatively stable output for large current flow.
Generally, an amplifier amplifies an input signal and provides the amplified signal at the output. The amplifier may be supplied with positive and negative voltages ranging from 5 to 15 volts. They are usually characterized by the amount of amplification and bandwidth of operation.
A switched-capacitor amplifier is a type of amplifier that operates in discrete steps. It is a clocked circuit that uses transistor switches to couple the capacitors in different configurations. For example, in a first phase of a clock cycle a voltage may be sampled by charging the capacitors of the switched-capacitor amplifier. In a second phase, the capacitors are coupled by transistor switches into a configuration to amplify the sampled voltage stored in the capacitors.
FIG. 1 shows a conventional common source switched-capacitor amplifier 100 with threshold voltage of transistors at approximately 0.7 to 0.9 volts. The input voltage 104 of the NMOS transistor amplifier 100 must be slightly higher than the threshold voltage of the NMOS transistor during a reset phase when the switch 102 is closed. However, when the supply voltage 106, V.sub.dd, is relatively low at about 1 to 1.5 volts, the input of the NMOS transistor amplifier 100 cannot be pulled up to a satisfactory voltage level for the capacitors to discharge.